Push-broom System Research Articles

Comparing Unmanned Aerial Multispectral and Hyperspectral Imagery for Harmful Algal Bloom Monitoring in Artificial Ponds Used for Fish Farming

This work aimed to assess the potential of unmanned aerial vehicle (UAV) multi- and hyper-spectral platforms to estimate chlorophyll-a (Chl-a) and cyanobacteria in experimental fishponds in Brazil. In addition to spectral resolutions, the tested platforms differ in the price, payload, imaging system, and processing. Hyperspectral airborne surveys were conducted using a push-broom system 276-band Headwall Nano-Hyperspec camera onboard a DJI Matrice 600 UAV. Multispectral airborne surveys were conducted using a global shutter-frame 4-band Parrot Sequoia camera onboard a DJI Phantom 4 UAV. Water quality field measurements were acquired using a portable fluorometer and laboratory analysis. The concentration ranged from 14.3 to 290.7 µg/L and from 0 to 112.5 µg/L for Chl-a and cyanobacteria, respectively. Forty-one Chl-a and cyanobacteria bio-optical retrieval models were tested. The UAV hyperspectral image achieved robust Chl-a and cyanobacteria assessments, with RMSE values of 32.8 and 12.1 µg/L, respectively. Multispectral images achieved Chl-a and cyanobacteria retrieval with RMSE values of 47.6 and 35.1 µg/L, respectively, efficiently mapping the broad Chl-a concentration classes. Hyperspectral platforms are ideal for the robust monitoring of Chl-a and CyanoHABs; however, the integrated platform has a high cost. More accessible multispectral platforms may represent a trade-off between the mapping efficiency and the deployment costs, provided that the multispectral cameras offer narrow spectral bands in the 660–690 nm and 700–730 nm ranges for Chl-a and in the 600–625 nm and 700–730 nm spectral ranges for cyanobacteria.

Improved surface displacement estimation through stacking cross-correlation spectra from multi-channel imagery

Studying sporadic and complex geophysical surface flows, like earthquakes or sea surface circulation, are challenging cases. If a satellite is able to image an event, it becomes essential to pull out as much information as possible. In this contribution we demonstrate a method to increase the coverage and signal-to-noise ratio for displacement estimation, making such surface flow estimates more complete. We leverage upon the redundant offset information acquired by multi-channel push-broom imagery. The individual cross-correlation spectra (cross power spectral density; Fourier transform of the cross-correlation function) of different spectral bands are averaged in the frequency domain before sub-pixel offset-estimation by phase-plane fitting. The method is demonstrated near Kaikōura, where in 2016 a surface rupture occurred. RapidEye data from two different dates were used to reconstruct the displacement. In addition, the circulation along the coast is estimated from data from a single date where multiple spectral bands were acquired within seconds which made stacking of cross-correlation spectra possible. The demonstrated methodology is applied to data from the already decommissioned RapidEye constellation, but can be adopted to other pushbroom systems, such as the Landsat legacy or Sentinel-2.

Design of a push-broom multi-beam radiometer mapping system based on toroidal SAT TV antenna

A functional diagram of a surface radiation mapping microwave radiometer system is presented. The system is proposed for use in agriculture, to map fields soil moisture, effectiveness of draining systems, etc. It has also many other useful applications, such as early detection of subsurface fires in peats and many more. Previous single-beam systems used a number of passes and long lasting flight times to obtain a map of the underlying surface. Some multi-beam systems used bulky electromechanical scanning antennae unsuitable for light aerial vehicles. Thus those systems were unaffordable to use for small agricultural businesses due to high production and operational costs. Authors propose low-cost microwave push-broom system that is based on a toroidal antenna and other budget components mass-produced for satellite television. Due to its low weight the complex can be mounted on an unmanned aerial vehicle. The microwave complex is designed to use six to fourteen radiometer channels of different bands on a single toroidal antenna. This allows to form the underlying surface map containing radiation data with limited number of aircraft passes significantly reducing survey time. The possibility of simultaneous operation in the Ku and C bands has been successfully investigated.

HYPERSPECTRAL PANORAMIC IMAGING

Abstract. Hyperspectral instruments are designed for the characterisation of planetary surfaces, oceans and the atmosphere. At the moment there are a number of aircraft systems and planned space missions. Examples for this are the hyperspectral missions for Earth remote sensing (EnMAP) and also for deep space and planetary missions (Mercury mission Bepi Colombo).There are basically two options for a hyperspectral system: Snapshot systems and scanning systems. This paper investigates a scanning hyperspectral push-broom systems. In most systems the input aperture is a long slit whose image is dispersed across a 2-D detector array, so that all points along a line in the scene are sampled simultaneously. To fill out the spatial dimension orthogonal to the slit, the scene is scanned across the entrance aperture. An ideal low cost hyperspectral scanning device analogue to push broom scanner is a 2D-detector with variable spectral filters, each filter being arranged perpendicular to the direction of flight.The biggest challenge is the mapping of the images of the individual spectral channels to each other (co-registration). The solution of the problem is the prerequisite for the use of this kind of hyperspectral cameras e.g. on board of an aircraft. Therefore, an investigation should focus on the procedure of data acquisition, correction and registration. In addition, an example showing the advantages of a possible application is explained.

Dual-mode pushbroom hyperspectral imaging using active system components and feed-forward compensation.

Among the methods developed for hyperspectral imaging, pushbroom spatial scanning stands out when it comes to achieving high spectral resolution over a wide spectral range. However, conventional pushbroom systems are usually realized using passive system components, which has limited their flexibility and adaptability and narrowed their application scenarios. In this work, we adopt a different approach to the design and construction of pushbroom systems based on using active internal components. We present a new system concept utilizing an internal line scanning unit and a rotating camera mechanism. This enables a dual-mode imaging system that allows switching between 2D spatial imaging and spectral imaging. The line scanning unit, which consists of a narrow slit mounted to a linear piezo motor, facilitates the spatial scanning of the target while eliminating the laborious relative motion between the target and the imaging system, which is needed in conventional spectrographs. A software is developed for the automation and synchronization of the active components, which enables a novel feed-forward compensation function to compensate the shift in the diffraction angle due to the scanning motion of the slit and provide higher flexibility in data acquisition.

Specim IQ: Evaluation of a New, Miniaturized Handheld Hyperspectral Camera and Its Application for Plant Phenotyping and Disease Detection.

Hyperspectral imaging sensors are promising tools for monitoring crop plants or vegetation in different environments. Information on physiology, architecture or biochemistry of plants can be assessed non-invasively and on different scales. For instance, hyperspectral sensors are implemented for stress detection in plant phenotyping processes or in precision agriculture. Up to date, a variety of non-imaging and imaging hyperspectral sensors is available. The measuring process and the handling of most of these sensors is rather complex. Thus, during the last years the demand for sensors with easy user operability arose. The present study introduces the novel hyperspectral camera Specim IQ from Specim (Oulu, Finland). The Specim IQ is a handheld push broom system with integrated operating system and controls. Basic data handling and data analysis processes, such as pre-processing and classification routines are implemented within the camera software. This study provides an introduction into the measurement pipeline of the Specim IQ as well as a radiometric performance comparison with a well-established hyperspectral imager. Case studies for the detection of powdery mildew on barley at the canopy scale and the spectral characterization of Arabidopsis thaliana mutants grown under stressed and non-stressed conditions are presented.

Volcanic Plume Elevation Model and its velocity derived from Landsat 8

Abstract In this paper we present a method to restitute the volcanic gas/ash Plume Elevation Model (PEM) from optical satellite imagery. As the volcanic plume is moving rapidly, conventional satellite based photogrammetric height restitution methods do not apply as the epipolar offset due to plume motion adds up to the one generated by the stereoscopic view. This is because there are time-lags of tens of seconds between conventional satellite stereoscopic acquisitions, depending on the stereo acquisition mode. Our method is based on a single satellite pass. We exploit the short time lag and resulting baseline that exist between the multispectral (MS) and the panchromatic (PAN) bands to jointly measure the epipolar offsets and the perpendicular to the epipolar (P2E) offsets. The first are proportional to plume height plus the offsets due to plume velocity in the epipolar direction. The second, are proportional to plume velocity in the P2E direction only. The latter is used to compensate the effect of plume velocity in the stereoscopic offsets by projecting it on the epipolar direction assuming a known plume direction, thus improving the height measurement precision. We apply the method to Landsat 8 data taking into account the specificities of the focal plane modules. We focus on the Holuhraun 2014 fissure eruption (Iceland). We validate our measurements against ground based measurements. The method has potential for detailed high resolution routine measurements of volcanic plume height/velocity. The method can be applied both to other multifocal plane modules push broom sensors (such as the ESA Sentinel 2) and potentially to other push-broom systems such as the CNES SPOT family and Pleiades.

Preparation and Applications of Low-loss As-S Chalcogenide Glass Fibers

High-purity As-S glasses were prepared by repetitive distillation and open-type dynamic distillation methods. The intensities of impurities' absorptions were obviously reduced at 2.9 mu m, 4 mu m and 6.3 mu m. Chalcogenide glass tubes with precise wall thickness and fine surface properties were prepared by tube-rotating technique and the fibers with core of 40 mu m, cladding of 50 mu m and error of 1% in diameter were drawn by rod-in-tube method. The fiber had good mechanical and optical performances, which general optical loss was less than 0.5 dB/m and the bending radius was less than 4 mm based on the bending test. The line-plane-switching fiber bundle which format was 64x9 quadrate array to implement 192x3 linear array was made from these chalcogenide glass and long-array infrared push-broom image was successfully demonstrated. The results prove that the fibers and bundle have excellent properties and push-broom infrared system based on the line-plane-switching fiber bundle being feasibility.

Prediction of white button mushroom (Agaricus bisporus) moisture content using hyperspectral imaging

Hyperspectral imaging is a non-contact, non-destructive technique that combines spectroscopy and imaging to extract information from a sample. This technology has recently emerged as a powerful technique for food analysis. In this study, the potential of hyperspectral imaging (HSI) to predict white button mushroom moisture content (MC) was investigated. Mushrooms were subjected to dehydration at 45 ± 1 °C for different time periods (0, 30, 60 and 120 min) to obtain representative samples at different moisture levels (93.40 ± 0.62%, 82.76 ± 2.11%, 73.20 ± 2.60% and 60.89 ± 4.32% wet basis [wb]). Hyperspectral images of the mushrooms were obtained using a pushbroom system operating in the wavelength range of 400–1000 nm. Hunter L, a and b colour values of the mushrooms were also measured. The average reflectance spectra of samples at different MC levels were obtained and Partial Least Square Regression (PLSR) models were built to predict mushroom moisture content. To reduce the spectral variability caused by factors unrelated to MC such as scattering effects and differences in sample height, different spectral pre-treatments were applied. The Standard Normal Variate (SNV) transformation was found to be the best approach among the wavelength range studied, resulting in the greatest reduction in Root Mean Square Error of Cross Validation (RMSECV) and Root Mean Square Error of Prediction (RMSEP) for a 4-component PLSR model. RMSECV of 5.50 (% wb) and RMSEP of 5.58 (% wb) were obtained for the calibration and test sets of data, respectively. Prediction maps were generated from hyperspectral data to show the predictive model performance at pixel level. This study shows the potential of hyperspectral imaging for prediction of mushroom moisture content in the studied wavelength range. The implemented method highlighted contrast between areas of different moisture content to achieve better knowledge of dehydration distribution over the mushroom surface.

COMPUTED TOMOGRAPHIC IMAGING SPECTROMETER (CTIS) AND A SNAPSHOT HYPERSPECTRAL IMAGER AND POLARIMETER

A Computed Tomographic Imagining Spectrometer (CTIS) is an imaging spectrometer system that acquires all the information required to reconstruct the data cube in a single integration time. This is compared to conventional systems such as whiskbroom systems, pushbroom systems, and filter wheel systems that requiring scanning in one or more coordinate direction. CTIS systems have been designed and tested in several different singular spectral bands as well as a dual band system. In addition to hyperspectral imaging spectrometers, CTIS systems have been used as an imaging spectropolarimeter and as a ranging imaging spectrometer. An imaging spectropolarimeter not only reconstructs the spectral content at every point in the scene of interest, but also provides the Stokes parameters at every point. So instead of just one data cube, we get four data cubes, one for each element of the Stokes vector. The ranging CTIS incorporates a LADAR system with the CTIS to provide the range information to targets in scene as well as the reconstructed data cube. The physical principles behind the CTIS system are presented as well as some of representative data from single band systems, the dual band proof of concept, the spectropolarimeter, and the ranging imaging spectrometer.

Analysis of computed tomographic imaging spectrometers I Spatial and spectral resolution

Computed tomographic imaging spectrometers measure the spectrally resolved image of an object scene in an entirely different manner from traditional whisk-broom or push-broom systems, and thus their noise behavior and data artifacts are unfamiliar. We review computed tomographic imaging spectrometry (CTIS) measurement systems and analyze their performance, with the aim of providing a vocabulary for discussing resolution in CTIS instruments, by illustrating the artifacts present in their reconstructed data and contributing a rule-of-thumb measure of their spectral resolution. We also show how the data reconstruction speed can be improved, at no cost in reconstruction quality, by ignoring redundant projections within the measured raw images.

High-throughput, multiplexed pushbroom hyperspectral microscopy

We describe a high-throughput hyperspectral microscope. The system replaces the slit of conventional pushbroom spectral imagers with a static coded aperture mask. We present the theoretical underpinnings of the aperture coded spectral engine and describe two proof-of-concept experimental implementations. Compared to a conventional pushbroom system, the aperture coded systems have 32 times greater throughput. Both systems have about a 1 nm spectral resolution over the spectral range of 550-665 nm. For the first design, the spatial resolution for the system is 5.4 microm while the spatial resolution for the second system ranges from 7.7 microm to 1.54 microm. We describe experimental results from proof-of-concept applications of the imager to hyperspectral microscopy.

Formulation of distortion error for the line-of-sight (LOS) vector adjustment model and its role in restitution of SPOT imagery

A recently developed line-of-sight (LOS) vector adjustment model is an effective geopositioning method for pushbroom images. A requirement for this method is that a predicted orbit be close to a true orbit. Although the method has proven very effective for SPOT 5, which has an accurate orbital control system, it was an open question whether or not the method could be effective for a satellite system with poor orbital accuracy. In this paper, a distortion error is defined and formulated as a common criterion for testing orbital requirements of a pushbroom system from the perspective of the LOS vector adjustment model. Using the distortion error, it is shown that a satellite system, even with poor locational accuracy comparable to SPOT 3–4, meets the orbital requirements as long as the maximum topographic height in the landscape is less than 4 km against the height level of the control points. A test was performed using three SPOT images and only five ground control points (GCPs). Results indicated that the root mean squared errors (RMSEs) of horizontal residual errors calculated from 30 check points were less than 10 m. The achieved accuracy of three-dimensional object-point determination was 6 m in the X -dimension and 7 m in both Y - and Z -dimensions. The conclusion can be drawn that the orbital requirements of this method can accommodate most space-borne systems, and therefore the LOS vector adjustment model is effective for SPOT and comparable pushbroom imagery.